Optically variable elements, such as holograms or interference layer elements, are gaining increasing importance in many areas for various reasons. As a rule these elements show different colors dependent on the viewing angle. However they can also contain different information which is recognizable solely at certain viewing angles. In the area of protection of a great variety of objects against forgery, falsification or other kinds of manipulation, holograms and similar diffraction structures have been applied in many ways, since the angledependent effects can be imitated only with disproportionately high effort. On the other hand, optically variable elements permit a fast visual check and offer good copy protection since the angle-dependent effects cannot be reproduced by a copying machine.
For the protection of security documents such as identity cards or bank notes, reflection holograms are frequently used. They are usually produced by means of special matrices which have a surface relief corresponding to the diffraction pattern of the hologram by embossing the surface relief in a layer of lacquer, subsequently metalizing it and a layer. The metalizing ensures the brilliance of the hologram so that it is visually well recognizable.
Various possibilities have been proposed for applying such holograms to security documents in a locally limited form. One of the most proven techniques is to prepare a separate, continuous transfer band from which the hologram is transferred to the document well-placed in the desired contour (DE-A1 33 08,831). The layers to be transferred are applied all over to a carrier substrate provided with an easily detachable separation layer in the reverse order to the one in which they are to exist later on the document. The uppermost layer is an adhesive layer, for example a hot-sealing adhesive layer. Via this adhesive layer the hot stamping band and the document are interconnected under the influence of heat and pressure (hot stamping). The carrier substrate of the transfer band can subsequently be removed effortlessly because of the separation layer. The definition of the surface areas to be transferred can take place either via the adhesive layer, which is printed in a certain pattern so that the layer structure to be transferred adheres to the document only in some places, or via the pressure stamp used during lamination, which in this case has a contour shape corresponding to the shape to be transferred, so that even if hot-sealing adhesive is applied over a large surface only those areas of the adhesive layer are activated on which the punch exerts pressure and heat (DE-A1 33 08 831). During the following removal of the transfer band the hologram layer structure comes off the transfer band in the area of the activated adhesive layer and adheres to the document. This presupposes, however, that the hologram layer structure is of very thin and relatively unstable design to guarantee a clean separation of the element from the continuous coating of the transfer band. This requirement conflicts with the general striving to make holograms as durable and resistant as possible.
To also do justice to the demand that the hologram be resistant to mechanical or other influences, it was proposed in the past to provide in the layer structure of the transfer band a thin resistant film layer which serves as a protective layer for the hologram after transfer to the final substrate (EP-A2 466 118). This guarantees sufficient protection of the hologram against mechanical and other loads, but the transfer of this layer structure to a receiving substrate requires additional method steps since the stable protective layer does not tear in the edge area of the activated adhesive layer upon removal of the transfer band and thus prevents extraction of the element. It is thus necessary to prestamp the contour of the desired surface element in the protective fllm before applying the hologram, so that it is possible to extract the hologram from the surrounding material during transfer.
An alternative possibility of transferring relatively durable hologram labels to documents by a modified transfer method is described in DE-A1 41 30 896. Here the carrier substrate first receives a cold adhesive layer over which the usual hologram layer structure is finally disposed in the order in which it is to exist on the document later, followed by a stable protective layer. To produce defined single elements, the layer structure is stamped without damaging the carrier substrate. The superfluous part of the laminar compound (grid) is removed from the carrier substrate so that only the spaced apart single labels are present on the carrier substrate. During transfer, the carrier substrate is removed and the label fastened to the document by means of the cold adhesive layer. Since the adhesive layer is covered by the carrier substrate, automated transfer of these labels is complicated and requires additional apparatus components which furter increase the already high production costs of hologram elements.
The invention is thus based on the problem of producing a laminar compound from durable and tear-resistant layers which contains diffraction structures and permits well-placed single elements to be applied to objects to be protected by the transfer method in a simple and cost-effective way.
The solution to this problem can be found in the independent claims. Developments are stated in the subclaims.